This invention relates to maximum likelihood detection of data recorded as analog signals representing a finite number of states, and, more particularly, to adapting maximum likelihood detection to handle variable data channels.
Maximum likelihood detection of data recorded as analog signals and detected from partial response samples is highly advantageous in magnetic disk drives, where the disks and heads are fixed and non-removable. The characteristics of the channel are fixed, including the particular disk media, the particular recording and read heads, the linear velocity and flying height between the disk media and the recording and read heads, and the recording and read electronics. The channel characteristics can be measured and, once known, tend to remain constant.
With constant channel characteristics, the detection capability of maximum likelihood detection can be designed so as to be highly error free. The received partial response samples are equalized so that the signals provided to the maximum likelihood detector tend to precisely match the expected signals.
Additionally, a specific code may be employed which maximizes the distances between the sensed states. Only limited changes are taken into account, such as differences in data rates between inner and outer tracks, minor servo offtrack operation, minor disk defects, and some head wear over time. Thus, a specific maximum likelihood detection circuit can be designed which is specific to the type of disk drive and which will have a low error rate at high recording densities. Further, such minor changes have been accommodated by employing digital FIR (finite impulse response) filters whose coefficients are programmable, thus changing the frequency response of the filters to better equalize the signal being read to match the maximum likelihood detector. Examples include, U.S. Pat. No. 5,321,559, Nguyen et al., U.S. Pat. No. 5,365,342, Abbott et al., and U.S. Pat. No. 5,442,760, Abbott et al.
It becomes more difficult to use such maximum likelihood detection with recording devices which have removable media.
Removable media devices tend to be mass storage devices which allow data to be recorded on portable media that is removed from the device and stored elsewhere, such as in the storage shelves of an automated data storage library, or in true archive storage outside of a drive or library on storage shelves or in boxes and other containers. The amount of data so stored quickly becomes very large and, if a new and upgraded portable media is introduced, there is a desire on the part of the user to resist re-recording all of the archived data onto the upgraded portable media. Hence, a backwards compatibility is typically required for removable media devices.
The characteristics of portable media thus vary between the types of media, but also tend to vary between ones of the same type of media, and within the same media.
Examples of removable media devices include optical disk and optical tape storage, which may be read-only, write-once, and rewritable media, and be different types of media, such as molded, magneto-optic and phase-change media.
Optical media is subject to variation from media to media in recorded data output characteristics based on the type of media, above, variation in media materials between manufacturers and over time, and between recording densities. Optical media is also subject to change as the recording moves between tracks, where the effective head to track speed may change, or as the recording moves from one side of the disk to another, where the effective head to disk surface angle may be tilted.
Another example of removable media devices includes magnetic tape recording, which have media to media variation based on different data densities on the same type of media, different types of media such as chromium-based, nickel-based, ferrous-based media, or between materials used by different manufacturers. Additionally, tape media may have differing thicknesses and therefore differing media to head (flying and contact) characteristics over the recording and read head, resulting in differing head to media spacings. The tape media may also fly at differing head to media spacings as the tape moves from one reel to the next and the tension or angle of the tape to the head varies. Further, the flying characteristics of the tape media may vary as the tape head is moved from one side of the tape to tracks at the other side of the tape.
Maximum likelihood detection in such differing circumstances is exceedingly difficult, and may require a different maximum likelihood detector for each circumstance.
The incorporated Hutchins et al. ""019 and ""020 applications provide maximum likelihood detectors which allow the programming, or setting, of numerical metric coefficients, adjusting the response of the maximum likelihood detector. The coefficients are preferably derived from the difference between metrics directly associating xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d states of the recorded signal. The coefficients are respectively applied to each digital sample to generate alternative metrics, and each respective alternative metric is compared to a previous metric. Based on the comparison, one of a plurality of provided metrics is selected which remains within defined positive and negative bounds. Then, the one of the finite number of states represented by the selected metric is identified, and in response to the identified one of the finite states, a maximum likelihood state detector is incremented to a maximum likelihood state dictated by the identified one of the finite states, the incremented maximum likelihood state detects the recorded analog signals.
The numerical metric coefficients of the maximum likelihood detector are based on sample xe2x80x9ccasesxe2x80x9d which are derived from sample outputs for expected waveforms of a particular media, and the metric coefficient numerical values are calculated for the expected sample outputs. Specifically, the sample outputs of the expected waveforms are determined, for example, by measuring a number of actual outputs for sample points of the waveforms, and calculating the mean values of each of the sample points, and the metric coefficient numerical values are calculated for the mean values of the sample outputs, thereby providing the numerical metric coefficients. The equations for deriving the metric coefficients minimize the mean squared error between the received signal and the ideal signal, which is the noise-free signal.
Once determined, the numerical metric coefficients are stored in a lookup table and selected when the portable media having the characteristics is identified as loaded into the device. A portable removable media may be identified externally by a label, or internally by reading a memory chip, reading a universal modulated code from the media, or the first few bytes of the media to be read may have more universal characters which are easily read which identify the media.
A media detector may comprise an optical reader or scanner for reading a label, a wireless interface for reading a chip, or may comprise logic associated with the read channel, and, the numerical metric coefficients are selected from the lookup table for the detected media.
Thus, the numerical coefficients for the specific media must be known in advance, and the predetermined numerical coefficients must be stored and available for use. Further, the characteristics of the specific media must not be variable, so that the same numerical coefficients may be used for the media without change.
However, as discussed above, optical media is subject to change as the recording moves between tracks, where the effective head to track speed may change, or as the recording moves from one side of the disk to another, where the effective head to disk surface angle may be tilted. Tape media may fly at differing head to media spacings as the tape moves from one reel to the next and the tension or angle of the tape to the. head varies. Further, the flying characteristics of the tape media may vary as the tape head is moved from one side of the tape to tracks at the other side of the tape.
Further, the specific portable removable media may not have an identification character that may be read to identify the media and select predetermined numerical coefficients.
Even if the media is identified, the read/write interface may change. For example, the single biggest change in a tape read/write interface is due to head wear. Tapes will cause abrasion to the head, eroding the MR read head material, whereas the metallic pole tips of the write element are not eroded as much. The net result is that the transfer function changes and differs from tape to tape depending on the age of a tape head when the data was written and the age of a same or different (different drive) tape head when the data is read.
It is an object of the present invention to provide adaptable maximum likelihood detection.
Another object of the present invention is to provide maximum likelihood detection for media whose precise characteristics are unknown so that there are no predetermined numerical coefficients.
Disclosed are an adapter and a method for adapting a programmable digital maximum likelihood detector to a variable channel output and a calibration system for calibrating a programmable digital maximum likelihood detector from unknown data in a known code at a variable channel output. The maximum likelihood detector is that of the incorporated Hutchins et al. ""019 and ""020 applications and detects digital samples of data recorded as analog signals representing a finite number of maximum likelihood states in accordance with a finite number of maximum likelihood sample-to-sample path cases. However, the digital samples represent the variable channel output of the recorded analog signals at a predetermined timing with respect thereto. In the incorporated Hutchins et al. applications, the detector comprises a sample input for receiving the digital samples of the recorded analog signals, and programming sources for providing numerical metric coefficients. Sample logic applies the programmed numerical metric coefficients to each of the digital samples to generate alternative metrics. Relational logic provides a previous metric which comprises a difference metric function of a previous digital sample derived from difference metrics representing the difference between metrics directly associating xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d states of the recorded signal. The one of the respective generated alternative metrics is selected which minimizes the mean squared error with respect to the previous metric and a maximum likelihood path memory identifies the maximum likelihood case represented by the selected metric, the set maximum likelihood case of the path memory state detecting the recorded analog signals.
The adapter comprises a detector identifying the maximum likelihood state corresponding to a digital sample of the recorded analog signals; an accumulator coupled to the detector for partially accumulating the detected digital sample with prior detected digital samples corresponding to the maximum likelihood state; and logic coupled to the accumulator and to the programming sources, employing the accumulated digital samples for the corresponding maximum likelihood state to determine and/or update the numerical metric coefficient for the maximum likelihood state.
The calibration system comprises a detector detecting digital samples of the recorded analog signals corresponding to one of the maximum likelihood states; an accumulator coupled to the detector for partially accumulating the detected digital samples for the corresponding maximum likelihood states; and logic coupled to the accumulator and to the programming sources, employing the accumulated digital samples for the corresponding maximum likelihood states to determine the numerical metric coefficients matching the digital samples to the one of the maximum likelihood states and setting the provided numerical metric coefficient to a value related to the determined matching numerical coefficient.
For a fuller understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.